Method for improving the adhesion of electrodeposited metal coatings



United States Patent 3,395,088 METHOD FOR IMPROVING THE ADHESION 0F ELECTRODEPOSITED METAL COATINGS Albert Edward Jackson, Killay, Swansea, England, assignor to The British Iron and Steel Research Association, London, England No Drawing. Filed May 14, 1965, Ser. No. 455,979 Claims priority, application Great Britain, May 15, 1964, 20,372/ 64 3 Claims. (Cl. 204-181) This invention is concerned with improvements in or relating to the formation of metal coatings.

In our British specification No. 884,797 we have described a process for forming a coating of, inter alia, aluminium on a metallic surface, which comprises electrophoretically depositing aluminium on the metallic surface from a suspension of finely divided aluminium in an organic liquid until a coating of the desired thickness is formed, drying the coated surface, compacting the coating, as by rolling the coated material, and then heating the coated material to improve the adhesion of the aluminium coating to the metallic surface.

In the case of substrates in the form of sheet, i.e. separate lengths of material, or in the form of strip or wire (which substrates are referred to as substrates in elongated form), the process is conveniently carried out by effecting the compaction stage and then either placing the sheets on top of one another to form a stack (in the case of sheet substrates) or coiling (in the case of strip and wire substrates), and carrying out the final heat treatment in the stack or coil as the case may be. In the case of aluminium coatings, this final heat treatment is preferably carried out at a temperature of from 250 C. to 500 C. for from hours to 30 minutes, the longer period being used for the lower temperature and vice versa.

We have found that while extremely adherent coatings are formed by this procedure in the top plies of the stack or the outer laps of the coil, the adhesion of the aluminium coating in the bottom and centre plies or the inner laps which are more tightly pressed together than the top plies or the outer laps is not so good and may be unsatisfactory.

We have further found that consistently excellent adhesion can be obtained in all parts of the stack or coil by placing the stack or coil in a gas-tight enclosure, reducing the pressure in the latter to evacuate gas from between adjacent layers of the coated substrate and then filling the enclosure with oxygen, these operations being carried out during the final heat treatment or immediately thereafter before the stack or coil has cooled to any appreciable extent from the temperature of the final heat treatment.

For this purpose, the heat treatment can be carried out in a vacuum furnace, that is a furnace which can be made gas-tight and evacuated to low pressures. In this case, the stack or coil is placed in the furnace, the latter is sealed and then evacuated while the stack or coil is heated up to the desired temperature. When the pressure has been reduced to a sufiiciently low level, say, 2 cm. Hg, oxygen is admitted to the furnace and the heat treatment continued for the period appropriate to the chosen temperature. Instead of carrying out a single evacuation and filling with oxygen, these two operations can be repeated in turn during the whole of the heat treatment.

In order that the invention may be more fully understood, the following example is given by way of illustration only:

EXAMPLE 1 Powdered aluminium was electrophoretically deposited 3,395,088 Patented July 30, 1968 ice on steel strip having a thickness of 0.025 inch by passing the latter, as the cathode, at a rate of 50 ft./min. through an electrophoresis tank containing a 10% (based on the total liquid) suspension of finely divided aluminium in industrial methylated spirit containing 1% water and 1 millimole/litre of nickel chloride. The cathode current density was 5 amps/ sq. ft. On leaving the electrophoresis tank, the coated strip was heated to 150 C. to dry it and then passed, While still hot, through compacting rolls which exerted a load of 5 tons/ inch width; after rolling the thickness of the aluminium coating was 25 microns.

The coated strip was then coiled on a mandrel of 18 inch diameter and the coil was heated to 450 C. over a period of 6 hours and held at this temperature for 30 minutes. After cooling to room temperature, samples of the coated strip were taken from different parts of the coil and were bent flat through 180 to test the adhesion of the aluminium coating. It was found that the samples taken from the outer laps of the coil had excellent adhesion and there was no delamination of the coating where the sample had been bent; the samples from the inner laps of the coil did not have such good adhesion, however, and there was delamination of the coating in the deformed areas of the samples.

A further similar steel strip was coated with aluminium, dried, rolled, and heat treated under the same conditions. At the end of the final heat treatment, however, the coil, still at 450 C., was placed in a gas-tight box, the latter was sealed and evacuated to about 2 cm. Hg. Oxygen was then admitted to the box and the coil allowed to cool to room temperature in an atmosphere of oxygen. Samples were taken from different parts of the coil and tested for adhesion of the coating as before. It was found that all the samples had consistently excellent adhesion as evidenced by no delamination occurring in the deformed areas of the samples.

I claim:

1. In a process for the coating of metal substrates in elongated form with aluminium by electrophoretically depositing finely divided aluminium on the substrate from a suspension thereof until a coating of the desired thickness is obtained, drying the coated substrate, rolling the coated substrate to compact the coating, and then heating the coated substrate in the form of a stack or coil to improve the adhesion of the aluminium coating to the substrate, the improvement which comprises placing the stack or coil in a gas-tight enclosure, reducing the pressure in the latter to evacuate gas from between adjacent layers of the coated substrate and then filling the enclosure with oxygen, said operations being carried out during the final heat treatment or immediately thereafter before the stack or coil has cooled substantially from the temperature of the final heat treatment.

2. A process according to claim 1, in which the steps of evacuating the enclosure and filling it with oxygen are carried out in repeated sequence during the course of the final heat treatment.

3. A process according to claim 1, in which the final heat treatment is carried out at a temperature of from 250 C. to 500 C.

References Cited UNITED STATES PATENTS 7/1966 Salt et a1. 204-181 7/1967 Jackson 2o4 1s1 

1. IN A PROCESS FOR THE COATING OF METAL SUBSTRATES IN ELONGATED FORM WITH ALUMINUM BY ELECTROPHORETICALLY DEPOSITING FINELY DIVIDED ALUMINIUM ON THE SUBSTRATE FROM A SUSPENSION THEREOF UNTIL A COATING OF THE DESIRED THICKNESS IS OBTAINED, DRYING THE COATED SUBSTRATE, ROLLING THE COATED SUBSTRATE TO COMPACT THE COATING, AND THEN HEATING THE COATED SUBSTRATE IN THE FORM OF A STACK OR COIL TO IMPROVE THE ADHESION OF THE ALUMINUM COATING TO THE SUBSTRATE, THE IMPROVEMENT WHICH COMPRISES PLACING THE STACK OR COIL IN A GAS-TIGHT ENCLOSURE, REDUCING THE PRESSURE IN THE LATTER TO EVACUATE GAS FROM BETWEEN ADJACENT LAYER OF THE COATED SUBSTRATE AND THEN FILLING THE ENCLOSURE WITH OXYGEN, SAID OPERATIONS BEING CARRIED OUT DURING THE FINAL HEAT TREATMENT OR IMMEDIATELY THEREAFTER BEFORE THE STACK OR COIL HAS COOLED SUBSTANTIALLY FROM THE TEMPERATURE OF THE FINAL HEAT TREATMENT. 